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| The term channel is mostly reserved to describe ion channels [9]. Pore and channel proteins do not require any conformational alteration in order for the solute entering from one side of the membrane to exit on the other side. They may undergo conformational changes, but these regulate whether they are open or closed to solute traffic and play no role in the mechanism of translocation per se. Two major groups of channels distinguish “voltage-gated” from “ligand-gated” channels. Channels are said to be voltage-gated if they are open or close in response to a change in the transmembrane potential. The open or closed state of ligand-gated channels is related to the binding of specific signal molecules.
Faciliated [sic] diffusion designates the carrier-mediated diffusion of solutes through the membrane. A carrier is a species that binds specific solutes and translocates them across the membrane. The role of carriers is usually to ensure the transport of polar or ionic substances that could not occur by simple diffusion across the lipid membrane. A carrier often presents a polar interior as binding site for the solute and exhibits a hydrophobic exterior compatible with the hydrocarbon region of the membrane. The carrier masks the hydrophilic nature of the polar solute and the complex thus formed can passively diffuse through the membrane. Like traditional passive diffusion processes, carrier-mediated transport can be driven either by a concentration or an electrochemical gradient of the transported species across the membrane. In all cases, the transport is down the gradient and thus does not require en [sic] energy input. This process is however saturable, since it is limited by the number of carriers, and usually exhibits specifically for solute structure. In some cases, it can also be inhibited by structural analogs to the transported solutes. I.3.3 Active transport Cells also have transport proteins that transfer solutes across the membrane against their electrochemical gradient. This process is called active transport because an input of energy is needed to bring it about. It is a saturable process and can be affected by competitive inhibitors. The energy to drive active transport may come from a number of sources. The most common source is the hydrolysis of ATP. Others include light energy [or the energy stored in ion gradients.] 9. Gennis, R. B. Biomembranes: Molecular Structure and Function; Springer-Verlag: Berlin, 1989. |
The term channel is mostly reserved to describe ion channels [19]. Pore and channel proteins do not require any conformational alteration in order for the solute entering from one side of the membrane to exit on the other side. They may undergo conformational changes, but these regulate whether they are open or closed to solute traffic and play no role in the mechanism of translocation per se. Two major groups of channels distinguish "voltage-gated" from "ligand-gated" channels. Channels are said to be voltage-gated if they open or close in response to a change in the transmembrane potential. The open or closed state of ligand-gated channels is related to the binding of specific signal molecules.
Facilitated diffusion Facilitated diffusion designates the carrier-mediated diffusion of solutes through the membrane. A carrier is a species that binds specific solutes and translocates them across the membrane. The role of carriers is usually to ensure the transport of polar or ionic substances that could not occur by simple diffusion across the lipid membrane. A carrier often presents a polar interior as binding site for the solute and exhibits a hydrophobic exterior compatible with the hydrocarbon region of the membrane. The carrier masks the hydrophilic nature of the polar solute and the complex thus formed can passively diffuse through the membrane. Like traditional passive diffusion processes, carrier-mediated transport can be driven either by a concentration or an electrochemical gradient of the transported species across the membrane. In all cases, the transport is down the gradient and thus does not require an energy input. This process is however saturable, since it is limited by the number of carriers, and usually exhibits specificity for solute structure. In some cases, it can also be inhibited by structural analogs to the transported solutes. [...] Active transport Cells also have transport proteins that transfer solutes across the membrane against their electrochemical gradient. This process is called active transport because an input of energy is needed to bring it about. It is a saturable process and can be affected by competitive inhibitors. The energy to drive active transport may come from a number of sources. The most common source is the hydrolysis of ATP. Others include light energy or the energy stored in ion gradients. [19] R. B. Gennis. Biomembranes: Molecular Structure and Function. Springer-Verlag, C. R. Cantor (Ed.), Berlin, 1989. |
The source is not mentioned. |
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